1. Field of the Invention
This invention relates generally to a low firing rate oxy-fuel burner. More particularly, this invention relates to a low firing rate oxy-fuel burner for use in such low firing rate applications as small reactors, retorts, crucibles, furnaces, molten glass distribution systems, feeders and refiners.
2. Background
Low firing rate burners (up to about 1,000,000 Btu/Hr) are useful in many types of heating applications. One particularly beneficial use of such burners is in molten glass distribution systems such as glass furnace forehearths.
In the production of glass objects, molten glass is produced in a glass melting furnace in which the raw materials for making glass are melted. The molten glass is passed from the glass melting furnace through a glass distribution system to the processing section or feeder for a forming machine in which the molten glass is processed into the desired glass shape or article.
The glass distribution system may include distribution channels, feeders and forehearths. Typical forehearths are designed to receive molten glass from the furnace and convey it to the glass processing section or machine and condition the glass as it is conveyed therethrough so that the glass is suitable for processing. Forehearths may generally include a refractory trough along which the molten glass flows and which is covered with an insulating roof.
A forehearth may also include two sections, a cooling section for receiving the molten glass and an equalizing section from which the molten glass exits into the processing section or feeder. The cooling section serves to cool or heat the molten glass to the required temperature for processing. However, it is known that the temperature of the glass is not homogeneous throughout its cross-section during travel through the cooling section. The glass tends to be cooler at the outside edges and hotter in the central portion due to the cooling effect of the side wall of the channel. For this reason, heating means such as gas burners are provided in the side of the cooling section to heat the glass. Cooling air may also be blown into the cooling section of the forehearth above the molten glass to cool the glass particularly in the central portion. By adjusting the rates of heating and cooling, homogeneity of the glass temperature throughout the cross section of the glass in the cooling section of the forehearth can be improved.
The molten glass flows from the cooling section into the equalizing section of the forehearth. The equalizing section serves to reheat the molten glass, particularly the outer surface, in the event it is too cool after leaving the cooling section. The equalizing section typically only uses heating provided by heating means such as burners disposed within the walls of that section of the forehearth. The temperature in the equalizing section is controlled independently of the temperature of the cooling section.
Traditional systems for heating glass in a forehearth system use combustion burners of a premix design in which the fuel, for example natural gas, and combustion air are premixed together in the correct stoichiometric ratio before they enter the burner. For example, U.S. Pat. No. 5,169,424 shows generally a forehearth structure in which gas-air burners provide heat to the molten glass flowing through the forehearth. U.S. Pat. No. 4,662,927 shows generally a glass distribution channel in which fuel/air burners provide a flame for heating the space above the flowing molten glass. U.S. Pat. No. 4,708,728 is directed to the use of premixed fuel-air burners for heating a glass distribution channel in which the burner has a capillary tube disposed coaxially therein and extending beyond the end of the burner for feeding oxidant into the fuel-air mixture.
However, the use of premixed air-fuel burners for glass distribution systems such as channels and forehearths is not totally satisfactory. Such burners provide very poor fuel efficiency and due to the high volume of combustion gasses, the associated emissions are very high. Further, the day-to-night changes in combustion air temperature causes fluctuations in overall flame temperature. Thus the process temperature may vary with the time of day or night. Most forehearth systems require precise temperature control of the glass, as small as 1.degree. or 2.degree. F. or less, which is difficult to attain with air-fuel burners.
Another disadvantage of the premixed air-fuel firing system is the very limited turndown ratio which, in turn, limits the level of control on the forehearth when responding to a temperature control signal to either increase or decrease fuel input. The turn-down ratio, i.e., the high firing rate of the burner divided by the low firing rate of the burner, for a premix air-fuel burner is only about 4:1 because velocities of the premixed air-gas flame which are too low may result in flashback, while velocities which are too high will blow the flame from the burner nozzle.
To overcome these problems, current practice is to replace the premixed air-fuel burners with 100% oxygen-fuel burners. For example, U.S. Pat. Nos. 5,500,030, 5,405,082, 5,256,058 and 5,199,866 show various arrangements of tube-in-tube design oxy-fuel burners for forehearth applications. However, the use of concentric fuel and oxygen nozzles presents several process and engineering problems for small firing rates.
Tube-in-tube oxy-fuel burners produce a fuel-rich flame core in the center which is surrounded by an annular oxygen flow. The fuel rich center core is allowed to mix with the outer oxygen stream very gradually. The delayed mixing between the co-flowing fuel and oxygen causes some of the fuel (natural gas) to crack or disassociate into soot particles. This soot rich fuel is later combusted with the oxygen to produce a very luminous flame. Most forehearth applications require a non-luminous flame which has minimum visible radiation to minimize any flame signature on the load. A luminous flame of distinct visible radiation can cause uneven heating and undesirable temperature gradients in the molten glass. The objective of the forehearth burner flame is to simply provide hot flame gases from non-luminous flame which are capable of providing a uniform heat flux of ultra violet and infrared wavelengths to the forehearth superstructure. Irradiation of this heat back to the molten glass provides for indirect uniform heating and not direct flame-to-glass heating done with the luminous flame. Uniform heating is necessary to provide the homogeneity in the molten glass bath, which results in a final glass product of good quality.
Additionally, the tube-in-tube design which utilizes a simple center hole for the natural gas nozzle and an annular passage for the oxygen nozzle provides a very long flame. From a forehearth design point, long flames are undesirable. Forehearths are relatively narrow channels. It is difficult to fire a burner with a very long flame length without impingement upon the opposite refractory wall or structure and possible damage thereto. Also, depending upon the design of the forehearth, long flames could eliminate the possibility of installing a burner at a corresponding location on the opposite side wall.
One option to overcome these problems is to design a burner with very high fuel and oxidant velocities to increase mixing and reduce flame length. However, this approach has practical limitations. The drilling of a smaller hole for the fuel nozzle to increase velocity could result in plugging due to soot particles or other particulate matter. The annular passage for the oxygen can not be practically reduced much smaller without causing problems in concentricity in the fuel and oxygen nozzles. A plugged fuel nozzle or misaligned or non-uniform annular oxygen passage can create an unstable flame of fluctuating heat-flux thus affecting the heating process and final product quality.
Moreover, the drilling of a smaller hole such as by laser drilling and using machined surfaces to create a very small annular passage, will result in a relatively high manufacturing cost for a burner. A large quantity of burners is needed in many forehearth applications and high manufacturing costs for a burner would make them prohibitively expensive for use in such applications. From an economics standpoint, a simple burner which is relatively inexpensive and which has the ability to generate a short, non-luminous flame is necessary for widespread use in forehearth applications.